had t-mobile for 10 years great overseas, not bad in local metro area's, but they going through freq, changes and even in a big city you can lose signal for hours, google "loss of signal with t-mobile" they best for traveling in remote area's is verizon, had them for 20+ years, i have driven the USA a lot, and to have coverage all the time you will need 2 carriers verizon and someone else. i got att this month, works great in the city, but waiting for a new phone to arrive that has all the att freq, the phone companies now have so many freq,you need to make sure your phone has the ones you need for your carrier and other carrier's freq for when you are roaming.
the new att was great until i left the city and had 70 miles of no coverage due phone being short a freq,
the expensive smart phones will have all the frequencies but others many not including the phone at the phone store. you can check on "gsmarena.com for phone specs. have not installed a signal booster, but it will not make up for 70miles of no tower.